specifying file descriptor number?

Question

My understanding was that one could not control the file descriptor (integer) assigned by the OS when opening a new file using open(). How then is it possible in a bash shell to assign a specific file descriptor using a command like

exec 5>&1

(I suppose I could find out by reading the bash sources...)

Answer 1

I believe you are right that sometimes file descriptors may already be in use. I got this from http://tldp.org/LDP/abs/html/io-redirection.html#FTN.AEN17716

"Using file descriptor 5 might cause problems. When Bash creates a child process, as with exec, the child inherits fd 5 (see Chet Ramey's archived e-mail, SUBJECT: RE: File descriptor 5 is held open). Best leave this particular fd alone."

The solutiont to this is specified in section 3.6 paragraph 2 of the bash manual.

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close.

For example

#!/bin/bash

exec {NEW_STDOUT}>&1
echo "Hello" >&$NEW_STDOUT
exec {NEW_STDOUT}>&-

Answer 2

See the dup2 Unix system call.